1. Field of the Invention
This invention relates to an apparatus for correcting a focal point according to a lens temperature in an optical focusing system of a camera such as a process camera, in which a distance between an object and a focusing lens and a distance between the focusing lens and a focusing screen vary only in limited ranges.
In contrast with an ordinary camera, the distance between the focusing lens and the object, and that between the focusing lens and the focusing screen change in very limited ranges in the process camera. Therefore it is possible to figure out, based on arithmetic expressions, the distance between the object and the focusing lens and that between the focusing lens and the focusing screen so as to produce images which are in best focus at a desired magnification, once the focal distance of the focusing lens is known. It is conceivable to develop autofocus units for the process cameras, in which when a desired magnification is inputted, the distances between the object, focusing lens and focusing screen will be automatically calculated and be corrected accordingly in order to produce clear and sharp images at the desired magnification.
The above-mentioned autofocus units differ from those for ordinary cameras. With autofocus units for ordinary cameras, a focusing operation is performed by feeding back focusing data according to an image being focused on the focusing screen. When a focal point exists within depth of focus, the image is considered to be in focus, and so strict focusing precision is not required. However the process camera is required to reproduce clearly and sharply an entire area of the original image in focus. Also, the objects are usually sheet-shaped in the case of the process camera.
The autofocus unit for the process camera mentioned above can assure production of well-focused sharp images. However there has been still some room for improvement in producing much clearer images with such a process camera. For example, the focal point tends to change depending upon temperatures of the focusing lens. To be more specific, as the temperature of the focusing lens varies, the size of the focusing lens itself and that of the lens barrel vary and result in the change of the focal point.
With the ordinary cameras, the autofocus unit performs focusing operation by feeding back focusing data while observing the image focused on the focusing screen. Therefore, the deviation of the focal point due to temperature variations is prevented. However, the autofocus unit for the process camera does not perform any feedback control for the focusing operation, and if the focal point changes depending upon varying temperatures of the focusing lens, this will cause images to be poorly reproduced. Such deterioration of the images becomes serious with the process camera while it is negligible with the ordinary cameras, since images which are considered poor in the field of the process camera would be acceptable as images having sufficient sharpness with the ordinary cameras. Therefore it is desirable to prevent deterioration of the images by some measures so as to obtain clearer images.